Carrier mobility in surface-channel transistors, apparatus made therewith, and systems containing same

ABSTRACT

A surface channel transistor is provided in a semiconductive device. The surface channel transistor is either a PMOS or an NMOS device. Epitaxial layers are disposed above the surface channel transistor to cause an increased bandgap phenomenon nearer the surface of the device. A process of forming the surface channel transistor includes grading the epitaxial layers.

TECHNICAL FIELD

A variety of electronic and optoelectronic devices use surface channeltransistors for microelectronic applications. Surface layers capable ofusing the properties of III-V materials may host a variety of highperformance electronic devices such as complementary metal oxidesemiconductor (CMOS) and quantum well (QW) transistors. The growth ofIII-V materials upon silicon substrates, however, presents manychallenges. Challenges involved with such devices include adequateshort-channel effect (SCE) and gate-length (Lg) scalability.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the manner in which embodiments are obtained, amore particular description of various embodiments briefly describedabove will be rendered by reference to the appended drawings. Thesedrawings depict embodiments that are not necessarily drawn to scale andare not to be considered to be limiting in scope. Some embodiments willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 a is a cross-section elevation of an integrated circuit deviceaccording to an example embodiment;

FIG. 1 b is a cross-section elevation of the integrated circuit devicedepicted in FIG. 1 a after further processing according to anembodiment;

FIG. 1 c is a cross-section elevation of the integrated circuit devicedepicted in FIG. 1 b after further processing according to anembodiment;

FIG. 2 is a band diagram comparison of a semiconductor device embodimentto another semiconductor device;

FIG. 3 is a cross-section elevation of a transistor apparatus accordingto an example embodiment;

FIG. 4 depicts graphical representations of the chemical similarity forthe sequences of layers that constitute epitaxial second structureembodiments;

FIG. 5 is a process flow diagram according to an embodiment; and

FIG. 6 is a schematic of an electronic system according to anembodiment.

DETAILED DESCRIPTION

Reference will now be made to the drawings wherein like structures maybe provided with like suffix reference designations. In order to showthe structures of various embodiments more clearly, the drawingsincluded herein are diagrammatic representations of integrated circuitstructures. Thus, the actual appearance of the fabricated integratedcircuit structures, for example in a photomicrograph, may appeardifferent while still incorporating the claimed structures of theillustrated embodiments. Moreover, the drawings may only show thestructures useful to understand the illustrated embodiments. Additionalstructures known in the art may not have been included to maintain theclarity of the drawings.

A surface channel transistor device utilizes bandgap engineering tocreate a sequence of compositional grading near thesemiconductor/dielectric interface. Disclosed techniques allow for thecarrier wavefunction to shift away from the dielectric interface toimprove dielectric density of interface traps (Dit) impurity scattering.Disclosed techniques provide useful mobility gains over conventionalsurface channel devices, particularly in systems with poor oxidequality, such as in III-V materials.

Disclosed embodiments address improving carrier mobility in a surfacechannel III-V device through bandgap engineering. A compositionallygraded sequence of thin layers is formed near the semiconductor surface.In an embodiment, the bandgap is graded from smaller to larger towardsthe surface. As a result, the quantum confinement of the carrier wavefunction is effectively weaker. Similarly the centroid of thewavefunction shifts the deeper into the semiconductor material, andfarther from the semiconductor/dielectric interface.

FIG. 1 a is a cross-section elevation of an integrated circuit device100 according to an example embodiment. The integrated circuit device100 may be used to form an NMOS or PMOS device on a semiconductivesubstrate 110. In an embodiment, the semiconductive substrate 110 is ahigh resistivity n or p-type (100) off-oriented Si substrate. In anembodiment, the semiconductive substrate 110 has a vicinal surface thatis prepared by off-cutting the semiconductive substrate 110 from aningot. In an embodiment, the semiconductive substrate 110 is off cut atan angle between 2 and 8 degrees towards the [110] direction to producea surface that may have terraces according to an embodiment. In anembodiment, different off-cut orientations are used. In an embodiment,the semiconductive substrate 110 is 4° off-cut silicon.

In an embodiment, the semiconductive substrate 110 is provided withoutan off-cut orientation. In any event, an off-cut semiconductivesubstrate 110 or other substrate preparation may provide for deviceisolation and may also reduce anti-phase domains in anti-phaseboundaries. The semiconductive substrate 110 may have a resistivity in arange from 1 ohm (Ω) to 50 kΩ.

A first structure 112 is disposed on the semiconductive substrate 110according to an embodiment. In an embodiment, the semiconductivesubstrate 110 may be separated from the first structure 112 such as by adielectric material such that the first structure 112 is a semiconductoron insulator (SOI) first structure 112. In an embodiment, the firststructure 112 is an epitaxial first structure 112 such as anIn_(0.7)Ga_(0.3)As material that has been formed on a semiconductivesubstrate 110 of InP. In an embodiment, the first structure 112 isconfigured to be a quantum well channel.

A first layer 114 of an epitaxial second structure 120 (see FIG. 1 b) isformed above (in the Z-direction) and on the epitaxial first structure112. Hereinafter, the first structure 112 is referred to as an epitaxialfirst structure 112, but it is also understood that it may not be anepitaxially formed first structure 112 such as an SOI structure.

FIG. 1 b is a cross-section elevation of the integrated circuit devicedepicted in FIG. 1 a after further processing according to anembodiment. The semiconductor device 101 has been processed with asequence of compositional grading above the epitaxial first structure112. A second layer 116 and a subsequent layer 186 are depicted, which,included with the first layer 114 are the epitaxial second structure120. Compositional grading of the first-through subsequent layers 114and 186, respectively, is accomplished in order to increase the bandgapnear the top 118 of the epitaxial second structure 120, which will bethe semiconductor/dielectric interface.

In an embodiment, formation of the epitaxial second structure 120 iscarried out by molecular beam epitaxy (MBE) such that atomic layers aresequentially grown above the epitaxial first structure 112. In anembodiment, MBE is carried out at temperatures between 400° C. and 500°C. In an embodiment, formation of the epitaxial second structure 120 iscarried out by chemical vapor deposition (CVD) such that layers aresequentially grown above the epitaxial first structure 112. In anembodiment, formation of the epitaxial second structure 120 is carriedout by metal-organic chemical vapor deposition (MOCVD) such that atomiclayers are sequentially grown above the epitaxial first structure 112.In an embodiment, formation of the epitaxial second structure 120 iscarried out by ultra-high chemical vapor deposition (UHCVD) such thatatomic layers are sequentially grown above the epitaxial first structure112. In an embodiment, formation of the epitaxial second structure 120is carried out by liquid-phase epitaxial deposition (LPE) such thatatomic layers are sequentially grown above the epitaxial first structure112.

Several quantitative layer embodiments are disclosed. In an embodiment,the number of layers that make up the epitaxial second structure 120 istwo including a first layer and a subsequent layer. In an embodiment,the number of layers that make up the epitaxial second structure 120 isthree including a first layer, a second layer, and a subsequent layer.In an embodiment, the subsequent layer 186 is up to a 30^(th) layer in asequence after the first layer 114. In an embodiment, the subsequentlayer 186 is up to a 25^(th) layer in a sequence after the first layer114. In an embodiment, the subsequent layer 186 is up to a 20^(th) layerin a sequence after the first layer 114. In an embodiment, thesubsequent layer 186 is up to a 15^(th) layer in a sequence after thefirst layer 114. In an embodiment, the subsequent layer 186 is up to a10^(th) layer in a sequence after the first layer 114. In an embodiment,the subsequent layer 186 is up to a 5^(th) layer in a sequence after thefirst layer 114.

Further to quantitative layer embodiments, the total thickness of anepitaxial second structure 120 may be between 20 nanometer (nm) and 100nm. Consequently, a 30-layer sequential composite may be between 20 nmthick and 100 nm thick. Similarly, a 5-layer sequential composite may bebetween 20 nm thick and 100 nm thick. Persons of ordinary skill in theart may now realize there are several permutations of layer number andepitaxial second structure thickness embodiments.

It should be appreciated that the quality of the epitaxial secondstructure 120 as a mechanism to shift the carrier wavefunction away froma dielectric interface above the semiconductor device 101 may beaffected by the specific structure of the epitaxial second structure 120and not just the number of layers and the ultimate thickness between thefirst layer 114 and the subsequent layer 186. Thus, both quantitativeeffects as in the number and thickness of graded sequential layers mayaffect performance, as well as qualitative effects as in the chemicalcomposition of each graded sequential layer.

FIG. 1 c is a cross-section elevation of the integrated circuit devicedepicted in FIG. 1 b after further processing according to anembodiment. The semiconductor device 102 is depicted with the sequenceof compositional grading above the epitaxial first structure 112 wherethe top 118 of the epitaxial second structure 120 is the subsequentlayer 186. FIG. 1 c is in simplified form according to an embodiment forillustrative purposes. It may now be understood the semiconductive firststructure such as the epitaxial first structure 112 is of asemiconductive first type and the subsequent layer of an epitaxialsecond structure such as the epitaxial second structure 120 is of asemiconductive second type. The gradation between the semiconductivefirst type and the semiconductive second type is set forth further inthis disclosure.

Compositional grading of the bandgap allows the carrier wave functionduring strong inversion (i.e. ON state condition) in the epitaxial firststructure 112 to shift deeper into the epitaxial first structure 112 andfarther from a dielectric interface that will be formed at the top 118of the epitaxial second structure 120. Consequently, interface impurityscattering from Dit at the dielectric interface is reduced as theepitaxial first structure 112 is used as the inversion region. Thedisclosed compositional grading embodiments may be used for III-Vmaterial semiconductive devices. In an embodiment, compositional gradingmay be used for silicon-based semiconductive devices.

In an embodiment, the semiconductive substrate 110 is anindium-phosphorus (InP) body and the epitaxial first structure 112 is aIII-V material of In_(0.7)Ga_(0.3)As. The epitaxial second structure 120is a sequence of epitaxially grown layers where the first layer 114 maybe In_(0.68)Ga_(0.32)As, the second layer 116 may beIn_(0.66)Ga_(0.34)As, and the subsequent layer 168 may beIn_(0.3)Ga_(0.7)As. As a consequence, the first layer 114 is chemicallymore similar to the epitaxial first structure 112 than to the subsequentlayer 168. “Chemically more similar” in this sense means thesemiconductive quality of a first structure 112 and a first layer 114 ofa second structure 120 are closer in behavior to each other than to asubsequent layer 168 of the second structure. It can be seen in thisexample embodiment that the compositions In_(0.7)Ga_(0.3)As (item 112)and In_(0.68)Ga_(0.32)As (item 114) are chemically more similar that thecomposition In_(0.3)Ga_(0.7)As (item 168). It can now be appreciatedthat a chemically more similar relationship among three givensemiconductive structures need not only be based upon an identicalthree-element chemistry with compositional variations. Where more thantwo layers exist in disclosed embodiments, “chemically more similar”also requires a gradation from the first layer 112 to the subsequentlayer 168. Selected gradation embodiments are set forth in connectionwith FIG. 5.

FIG. 2 is a band diagram 200 comparison of a semiconductor deviceembodiment to another semiconductor device. A Schrödinger-Poissonsolution of a spatial redistribution of a confined carrier population invarious subbands is depicted such as the epitaxial first structure 112depicted in FIG. 1 b at a gate voltage, V_(g)=0V. The band diagram 200shows a surface channel In_(0.7)Ga_(0.3)As transistor (upper graphic)compared with a transistor that has an epitaxial second structure with agradation (lower graphic). The epitaxial second structure beginschemically similar to the In_(0.7)Ga_(0.3)As material, but results witha In_(0.3)Ga_(0.7)As subsequent layer at the top (e.g. subsequent layer168 at the top 118 in FIG. 2 b). It can be seen that the wavefunctionhas shifted about 50 Angstrom from the dielectric interface. The bandgap 222 for the graded semiconductive device embodiment is about 50 Ålarger than the bandgap 220 for the other semiconductive device.

In an embodiment, graded doping with sequences of layers is carried outaccording to the structure depicted in the Table. It can be seen in theTable, that a 21-layer graded doping is formed. The element amounts setforth in the columns represent stoichiometric percentages based on anarsenic stoichiometery of unity.

Layer number In Ga As subsequent 0.3 0.7 1 20 0.32 0.68 1 19 0.34 0.66 118 0.36 0.64 1 17 0.38 0.62 1 16 0.4 0.6 1 15 0.42 0.58 1 14 0.44 0.56 113 0.46 0.54 1 12 0.48 0.52 1 11 0.50 0.5 1 10 0.52 0.48 1  9 0.54 0.461  8 0.56 0.44 1  7 0.58 0.42 1  6 0.6 0.4 1  5 0.62 0.38 1  4 0.64 0.361  3 0.66 0.64 1  2 0.68 0.32 1 first 0.7 0.3 1In an embodiment, the graded epitaxial second structure depicted in theTable has a thickness of 80 nm. In an embodiment, the graded epitaxialsecond structure depicted in the Table has a thickness of 600 nm.

FIG. 3 is a cross-section elevation of a transistor apparatus 300according to an example embodiment. A semiconductive substrate 310carries an epitaxial first structure 312. In an embodiment, theepitaxial first structure is a III-V semiconductive material to become aquantum well channel. An epitaxial second structure 320 is disposedabove and on the epitaxial first structure 312. The epitaxial secondstructure 320 includes at least a first layer 314 and a subsequent layer386. As illustrated according to an embodiment, the epitaxial secondstructure 320 includes a first layer 314, a second layer 316, and asubsequent layer 386. In an embodiment the subsequent layer 386 is up toa 30^(th) layer in a sequence after the first layer 314 according to theseveral embodiments set forth similarly to those disclosed for FIG. 1 b.

The transistor apparatus 300 may be processed with source- and drainwells 322 and 324, respectively. In an embodiment, the source well 322may have a tip 321 that may be formed before completion of thetransistor. Similarly in an embodiment, the drain well 324 may also havea tip 323.

A high-k gate dielectric film 326 is formed above and on the epitaxialsecond structure 320. The interface that forms between the high-k gatedielectric film 326 and the epitaxial second structure 320 is thesemiconductor/dielectric interface. In an embodiment, the high-k gatedielectric film 326 has a thickness from 20 Å to 60 Å. In an embodiment,the high-k dielectric film 326 is hafnium oxide (HfO₂). In anembodiment, the high-k dielectric film 326 is alumina (Al₂O₃). In anembodiment, the high-k dielectric film 326 is tantalum pentaoxide(Ta₂O₅). In an embodiment, the high-k dielectric film 326 is zirconiumoxide (ZrO₂). In an embodiment, the high-k dielectric film 326 islanthanum aluminate (LaAlO₃). In an embodiment, the high-k dielectricfilm 326 is gadolinium scandate (GdScO₃). As used herein, the phrase“high-k” refers to materials having a dielectric constant, k, greaterthan that of silicon dioxide, that is, greater than about 4.

The transistor apparatus 300 is further processed by forming a contactlayer 328 above the epitaxial second structure 320. The contact layer328 provides source- and drain contact structures with low contactresistance. In an embodiment, the contact layer 328 is formed ofIn_(x)Ga_(1-x)As. For an NMOS transistor 300 the contact layer 328 is n+doped. The contact layer 328 may also be n++ doped. In an embodiment,the contact layer 328 is doped by grading, starting with silicon dopedwith In_(0.53)Ga_(0.47)As, and proceeding from In_(x)Ga_(1-x)As fromx=0.53 to 1.0 such that grading terminates with InAs. For a PMOStransistor 300, the contact layer 328 is p+ doped. In an embodiment,graded doping is done with a p+ doping gradient. The contact layer 328may have a thickness between 10 nm and 30 nm according to an embodiment.The contact layer 328 has a thickness of 20 nm according to anembodiment.

A metal gate 330 is formed above and on the high-k gate dielectric film326. In an embodiment, the metal gate 330 is a titanium (Ti) material.In an embodiment, the metal gate 330 is a platinum (Pt) material. In anembodiment, the metal gate 330 is a gold (Au) material. In anembodiment, the metal gate 330 is a combination of at least two oftitanium, platinum, and gold. In an embodiment, the metal gate 330 has athickness (Z-direction) from 60 Å to 140 Å. In an embodiment, the metalgate 330 has a thickness of 100 Å. In an embodiment, the high-k gatedielectric film 326 has a thickness of 100 Å and the metal gate 330 hasa thickness of 100 Å. Further gate structure includes spacers 332 and adielectric cap layer 334. In an embodiment, the transistor 300 isisolated by shallow-trench isolation 336 structures.

FIG. 4 depicts graphical representations of the chemical similarity forthe sequences of layers that constitute epitaxial second structureembodiments. In an embodiment, the epitaxial second structure isdisposed on the epitaxial first structure and the epitaxial secondstructure is a sequential composite including a first layer and asubsequent layer. In an embodiment, the epitaxial second structureincludes graded In_(x)Ga_(1-x)Sb and the subsequent layer is up to a30th layer after the first layer according to any of the severaldisclosed embodiments.

In FIG. 4 a, a sequential composite of layers is represented that have alinear composition gradient. The abscissa coordinate (Z-coordinate) andthe ordinate coordinate (other linear composition gradient coordinate)are given in arbitrary units. At the origin of the graphicrepresentation, the concentration is represented as the ordinate, andthe distance from the epitaxial first structure to the top (e.g. top 118in FIG. 1 b) of the epitaxial second structure as the abscissa. In anembodiment, the semiconductive substrate is an InP material, theepitaxial first structure includes In_(0.7)Ga_(0.3)As, and the epitaxialsecond structure is up to a 30^(th) layer after the first layer thatincludes In_(0.3)Ga_(0.7)As. The concentration gradient in the epitaxialsecond structure is linear from the first layer to the subsequent layer.Other disclosed layer count embodiments are also useful second structureembodiments.

In FIG. 4 b, a sequential composite of layers is represented that have apositive (increasing) exponential composition gradient. The ordinate andabscissa coordinates are given in arbitrary units. At the origin of thegraphic representation, the concentration is represented as theordinate, and the distance from the epitaxial first structure to the topof the epitaxial second structure is represented as the abscissa. In anembodiment, the semiconductive substrate is an InP material, theepitaxial first structure includes In_(0.7)Ga_(0.3)As, and the epitaxialsecond structure is up to a 30^(th) layer after the first layer thatincludes In_(0.3)Ga_(0.7)As. The concentration gradient is increasingexponentially beginning at the first layer and ending at the subsequentlayer. Other disclosed layer count embodiments are also useful secondstructure embodiments.

In FIG. 4 c, a sequential composite of layers is represented that have anegative (decreasing) exponential composition gradient. The ordinate andabscissa are given in arbitrary units. At the origin of the graphicrepresentation, the concentration is represented as the ordinate, andthe distance from the epitaxial first structure to the top of theepitaxial second structure is represented as the abscissa. In anembodiment, the semiconductive substrate is an InP material, theepitaxial first structure includes In_(0.7)Ga_(0.3)As, and the epitaxialsecond structure is up to a 30^(th) layer after the first layer thatincludes In_(0.3)Ga_(0.7)As. The concentration gradient is decreasingexponentially beginning at the first layer and ending at the subsequentlayer. Other disclosed layer count embodiments are also.

In FIG. 4 d, a sequential composite of layers is represented that have apositive and a negative exponential composition gradient that includesan inflection. The ordinate and abscissa are given in arbitrary units.At the origin of the graphic representation, the concentration isrepresented as the ordinate, and the distance from the epitaxial firststructure to the top of the epitaxial second structure is represented asthe abscissa. In an embodiment, the semiconductive substrate is an InPmaterial, the epitaxial first structure includes In_(0.7)Ga_(0.3)As, andthe epitaxial second structure is up to a 30^(th) layer after the firstlayer that includes In_(0.3)Ga_(0.7)As. The concentration gradient isfirst increasing beginning at the first layer and then decreasing afteran inflection to end at the subsequent layer. Other disclosed layercount embodiments are also useful second structure embodiments.

In FIG. 4 e, a sequential composite of layers is represented that have apositive and a negative exponential composition gradient that includesan inflection and an asymptote. The ordinate and abscissa are given inarbitrary units. At the origin of the graphic representation, theconcentration is represented as the ordinate, and the distance from theepitaxial first structure to the top of the epitaxial second structureis represented as the abscissa. In an embodiment, the semiconductivesubstrate is an InP material, the epitaxial first structure includesIn_(0.7)Ga_(0.3)As, and the epitaxial second structure is up to a30^(th) layer after the first layer that includes In_(0.3)Ga_(0.7)As.The concentration gradient is first increasing beginning with the firstlayer and then decreasing after an inflection and an asymptote to end atthe subsequent layer. Other disclosed layer count embodiments are alsouseful second structure embodiments.

In FIG. 4 f, a sequential composite of layers is represented that have anegative and a positive exponential composition gradient that includesan inflection. The ordinate and abscissa are given in arbitrary units.At the origin of the graphic representation, the concentration isrepresented as the ordinate, and the distance from the epitaxial firststructure to the top of the epitaxial second structure is represented asthe abscissa. In an embodiment, the semiconductive substrate is an InPmaterial, the epitaxial first structure includes In_(0.7)Ga_(0.3)As, andthe epitaxial second structure is up to a 30^(th) layer after the firstlayer that includes In_(0.3)Ga_(0.7)As. The concentration gradientbeginning at the first layer is first decreasing exponentially and thenincreasing exponentially after an inflection to end at the subsequentlayer. Other disclosed layer count embodiments are also useful secondstructure embodiments.

In FIG. 4 g, a sequential composite of layers is represented includes aninflection and a relational concentration where the concentrationreverses to a degree to reach the subsequent layer. It should be clearthat several variations of linear, exponential, asymptotic, andreverse-relational sequences may be combined according to specificdesign needs.

Various structures may be used for a surface-channel transistor.Selected chemistries may include III-V semiconductive material. In anexample embodiment, the epitaxial second structure is 80 nm thick,includes 20 layers, and exhibits a sequential grading profile depictedin FIG. 4 a. In an example embodiment, the epitaxial second structure is60 nm thick, includes 30 layers, and exhibits a sequential gradingprofile depicted in FIG. 4 e.

In an embodiment, the transistor apparatus includes the epitaxial secondstructure with a sequential composite of graded In_(x)Ga_(1-x) accordingto any of the embodiments depicted in FIG. 4 and their art-recognizedequivalents In an embodiment, the transistor apparatus includes theepitaxial second structure with a sequential composite of gradedIn_(x)Sb_(1-x) according to any of the embodiments depicted in FIG. 4and their art-recognized equivalents. Similarly, each sequentialcomposition gradation embodiment may be combined with each disclosedquantitative layer number and overall thickness embodiment.

In an embodiment, the transistor apparatus includes the epitaxial firststructure of In_(0.7)Ga_(0.3)As and the subsequent layer includes asequential composite of In_(0.3)Ga_(0.7)As according to any of theembodiments depicted in FIG. 4 and their art-recognized equivalents.Similarly, each sequential composition gradation embodiment may becombined with each disclosed quantitative layer number and overallthickness embodiment.

In an embodiment, the transistor apparatus includes the epitaxial firststructure of Si_(x)Ge_(1-x) and the subsequent layer is a sequentialcomposite on a linear progression up to a 30^(th) layer and is Si or isapproaching Si in composition from SiGe. The various embodiments ofsequential composite gradation as depicted in FIG. 4 are also applied tothis chemistry. Similarly, each sequential composition gradationembodiment may be combined with each disclosed quantitative layer numberand overall thickness embodiment.

In an embodiment, the transistor apparatus includes the epitaxial firststructure made of Al_(0.3)In_(0.7)Sb and the subsequent layer is asequential composite of up to a 30^(th) layer after the first layer, andthe subsequent layer achieves or approaches Al_(0.7)In_(0.3)Sb. Thevarious embodiments of sequential composite gradation as depicted inFIG. 4 are also applied to this chemistry. Similarly, each sequentialcomposition gradation embodiment may be combined with each disclosedquantitative layer number and overall thickness embodiment.

In an embodiment, the transistor apparatus includes the epitaxial firststructure made of Al_(0.3)Ga_(0.7)As, the subsequent layer is up to a30^(th) layer after the first layer, and the subsequent layer includesAl_(0.7)Ga_(0.3)As. The various embodiments of sequential compositegradation as depicted in FIG. 4 are also applied to this chemistry.Similarly, each sequential composition gradation embodiment may becombined with each disclosed quantitative layer number and overallthickness embodiment.

FIG. 5 is a process flow diagram 500 according to example embodiments.

At 510, the process includes forming a semiconductive first structure.In an example embodiment, an epitaxial first structure ofIn_(0.7)Ga_(0.3)As is formed above a semiconductive substrate of InP asdepicted in FIG. 1 a.

At 520, the process includes forming a first layer of an epitaxialsecond structure above and on the semiconductive first structure. In anexample embodiment, a first layer of In_(0.68)Ga_(0.32)As is formedabove and on an epitaxial first structure of In_(0.7)Ga_(0.3)As asdepicted in FIG. 1 b.

At 530, the process includes forming a subsequent layer of the epitaxialsecond structure above the first layer. In an example embodiment, asubsequent layer of In_(0.3)Ga_(0.7)As is formed above the first layerof In_(0.68)Ga_(0.32)As.

At 532, the process includes forming a linear composition gradientbetween the first layer and the subsequent layer. In an exampleembodiment, a composition gradient that qualitatively matches theillustration of FIG. 5 a is achieved.

At 534, the process includes forming a non-linear (e.g. exponential)composition gradient between the first layer and the subsequent layer.In an example embodiment, a composition gradient that qualitativelymatches the illustration of FIG. 5 b is achieved. In an exampleembodiment, a composition gradient that qualitatively matches theillustration of FIG. 5 c is achieved. In an example embodiment, acomposition gradient that qualitatively matches the illustration of FIG.5 d is achieved. In an example embodiment, a composition gradient thatqualitatively matches the illustration of FIG. 5 e is achieved. In anexample embodiment, a composition gradient that qualitatively matchesthe illustration of FIG. 5 f is achieved. In an example embodiment, acomposition gradient that qualitatively matches the illustration of FIG.5 g is achieved.

It may now be understood that the nonlinear composition gradientembodiments may be discrete approximations of nonlinear compositiongradients, for example, a 30-layer non-linear composition gradient maybe viewed by fitting a curve to the changing chemistry in each layer. Inan embodiment, two linear composition gradients may be combined to becurve fit to appear similar to the qualitative effects of FIG. 4 b orFIG. 4 c.

At 540, the process includes forming a transistor apparatus that usesthe semiconductive first structure as an inversion region. In an exampleembodiment, a transistor apparatus 300 is formed with an epitaxial firststructure 312 and an epitaxial second structure 320 as depicted in FIG.3.

FIG. 6 is a schematic of an electronic system 600 according to anembodiment. The electronic system 600 as depicted can embody atransistor apparatus with a compositionally graded sequence of thinlayers embodiments disposed above a semiconductive first structure asset forth in this disclosure. In an embodiment, the electronic system600 is a computer system that includes a system bus 620 to electricallycouple the various components of the electronic system 600. The systembus 620 is a single bus or any combination of busses according tovarious embodiments. The electronic system 600 includes a voltage source630 that provides power to the integrated circuit 610. In someembodiments, the voltage source 630 supplies current to the integratedcircuit 610 through the system bus 620.

The integrated circuit 610 is electrically coupled to the system bus 620and includes any circuit, or combination of circuits according to anembodiment. In an embodiment, the integrated circuit 610 includes aprocessor 612 that can be of any type. As used herein, the processor 612may mean any type of circuit such as, but not limited to, amicroprocessor, a microcontroller, a graphics processor, a digitalsignal processor, or another processor. Other types of circuits that canbe included in the integrated circuit 610 are a custom circuit or anapplication-specific integrated circuit (ASIC), such as a communicationscircuit 614 for use in wireless devices such as cellular telephones,pagers, portable computers, two-way radios, and similar electronicsystems. In an embodiment, the processor 610 includes on-die memory 616such as static random-access memory (SRAM). In an embodiment, theprocessor 610 includes embedded on-die memory 616 such as embeddeddynamic random-access memory (eDRAM) that can be a cache memory for theprocessor.

In an embodiment, the electronic system 600 also includes an externalmemory 640 that in turn may include one or more memory elements suitableto the particular application, such as a main memory 642 in the form ofRAM, one or more hard drives 644, and/or one or more drives that handleremovable media 646, such as diskettes, compact disks (CDs), digitalvariable disks (DVDs), flash memory keys, and other removable mediaknown in the art. The various memory functionalities can containtransistor apparatus with a compositionally graded sequence of thinlayers embodiments disposed above a semiconductive first structure.

In an embodiment, the electronic system 600 also includes a displaydevice 650, an audio output 660. In an embodiment, the electronic system600 includes a controller 670, such as a keyboard, mouse, trackball,game controller, microphone, voice-recognition device, or any otherdevice that inputs information into the electronic system 600.

As shown herein, the integrated circuit 610 can be implemented in anumber of different embodiments, including a transistor apparatus with acompositionally graded sequence of thin layers embodiments disposedabove a semiconductive first structure, an electronic system, a computersystem, one or more methods of fabricating an integrated circuit, andone or more methods of fabricating an electronic assembly that includesa transistor apparatus with a compositionally graded sequence of thinlayers embodiments disposed above a semiconductive first structure asset forth herein in the various embodiments and their art-recognizedequivalents. The elements, materials, geometries, dimensions, andsequence of operations can all be varied to suit particular transistorapparatus with a compositionally graded sequence of thin layersembodiments disposed above a semiconductive first structure.

Although a processor chip and a memory chip may be mentioned in the samesentence, it should not be construed that they are equivalentstructures. Reference throughout this disclosure to “one embodiment” or“an embodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. The appearance ofthe phrases “in one embodiment” or “in an embodiment” in various placesthroughout this disclosure are not necessarily all referring to the sameembodiment. Furthermore, the particular features, structures, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Terms such as “upper” and “lower” “above” and “below” may be understoodby reference to the illustrated X-Z coordinates, and terms such as“adjacent” may be understood by reference to X-Y coordinates or to non-Zcoordinates.

The Abstract is provided to comply with 37 C.F.R. §1.72(b) requiring anabstract that will allow the reader to quickly ascertain the nature andgist of the technical disclosure. It is submitted with the understandingthat it will not be used to interpret or limit the scope or meaning ofthe claims.

In the foregoing Detailed Description, various features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments of the inventionrequire more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive subject matter lies in lessthan all features of a single disclosed embodiment. Thus the followingclaims are hereby incorporated into the Detailed Description, with eachclaim standing on its own as a separate preferred embodiment.

It will be readily understood to those skilled in the art that variousother changes in the details, material, and arrangements of the partsand method stages which have been described and illustrated in order toexplain the nature of this invention may be made without departing fromthe principles and scope of the invention as expressed in the subjoinedclaims.

1. A transistor apparatus comprising: a semiconductive substrate; anepitaxial first structure of a semiconductive first type disposed on thesemiconductive substrate; and an epitaxial second structure disposed onthe epitaxial first structure, wherein the epitaxial second structure isa composite including: a first layer of a semiconductive second typedisposed above and on the epitaxial first structure; a second layer of asemiconductive third type disposed above and on the first layer; and asubsequent layer of a semiconductor subsequent type disposed above thefirst layer of a semiconductive second type, wherein the first layer ischemically more similar to the epitaxial first structure than to thesubsequent layer, and wherein the first layer, the second layer, and thesubsequent layer form a gradation.
 2. The transistor apparatus of claim1, wherein the epitaxial second structure includes gradedIn_(x)Ga_(1-x)Sb, wherein the subsequent layer is up to a 30th layerafter the first layer.
 3. The transistor apparatus of claim 1, whereinthe epitaxial second structure includes graded In_(x)Ga_(1-x), whereinthe subsequent layer is up to a 30th layer after the first layer.
 4. Thetransistor apparatus of claim 1, wherein the epitaxial second structureincludes graded In_(x)Sb_(1-x), wherein the subsequent layer is up to a30th layer after the first layer.
 5. The transistor apparatus of claim1, wherein the semiconductive substrate includes indium phosphorus(InP), wherein the epitaxial first structure includesIn_(0.7)Ga_(0.3)As, wherein the subsequent layer is up to a 30th layerafter the first layer, and wherein the subsequent layer includesIn_(0.3)Ga_(0.7)As.
 6. The transistor apparatus of claim 1, wherein thesemiconductive substrate includes indium phosphorus (InP), wherein theepitaxial first structure includes In_(0.7)Ga_(0.3)As, wherein thesubsequent layer is up to a 30th layer after the first layer, whereinthe subsequent layer includes In_(0.3)Ga_(0.7)As, and wherein thereexists a linear InGa gradation between the first layer and thesubsequent layer.
 7. The transistor apparatus of claim 1, wherein thesemiconductive substrate includes indium phosphorus (InP), wherein theepitaxial first structure includes In_(0.7)Ga_(0.3)As, wherein thesubsequent layer is up to a 30th layer after the first layer, whereinthe subsequent layer includes In_(0.3)Ga_(0.7)As, and wherein thereexists an exponential InGa gradation between the first layer and thesubsequent layer.
 8. The transistor apparatus of claim 1, wherein thesemiconductive substrate includes indium phosphorus (InP), wherein theepitaxial first structure includes In_(0.7)Ga_(0.3)As, wherein thesubsequent layer is up to a 30th layer after the first layer, whereinthe subsequent layer includes In_(0.3)Ga_(0.7)As, and wherein thereexists an exponential InGa gradation with an inflection between thefirst layer and the subsequent layer.
 9. The transistor apparatus ofclaim 1, wherein the semiconductive substrate includes indium phosphorus(InP), wherein the epitaxial first structure includesIn_(0.7)Ga_(0.3)As, wherein the subsequent layer is up to a 30th layerafter the first layer, wherein the subsequent layer includesIn_(0.3)Ga_(0.7)As, and wherein there exists an exponential InGagradation with an inflection and an asymptote between the first layerand the subsequent layer.
 10. The transistor apparatus of claim 1,wherein the epitaxial first structure includes Si_(x)Ge_(1-x), whereinthe subsequent layer is up to a 30th layer after the first layer and isSi, and wherein there exists a linear SiGe gradation between the firstlayer and the subsequent layer.
 11. The transistor apparatus of claim 1,wherein the epitaxial first structure includes Si_(x)Ge_(1-x), whereinthe subsequent layer is up to a 30th layer after the first layer and isSi, and wherein there exists an exponential SiGe gradation between thefirst layer and the subsequent layer.
 12. The transistor apparatus ofclaim 1, wherein the epitaxial first structure includes Si_(x)Ge_(1-x),wherein the subsequent layer is up to a 30th layer after the first layerand is Si, and wherein there exists an exponential SiGe gradation withan inflection between the first layer and the subsequent layer.
 13. Thetransistor apparatus of claim 1, wherein the epitaxial first structureincludes Si_(x)Ge_(1-x), wherein the subsequent layer is up to a 30thlayer after the first layer and is Si, and wherein there exists anexponential SiGe gradation with an inflection and an asymptote betweenthe first layer and the subsequent layer.
 14. The transistor apparatusof claim 1, wherein the epitaxial first structure includesAl_(0.3)In_(0.7)Sb, wherein the subsequent layer is up to a 30th layerafter the first layer, and wherein the subsequent layer includesAl_(0.7)In_(0.3)Sb.
 15. The transistor apparatus of claim 1, whereinepitaxial first structure includes Al_(0.3)In_(0.7)Sb, wherein thesubsequent layer is up to a 30th layer after the first layer, whereinthe subsequent layer includes Al_(0.7)In_(0.3)Sb, and wherein thereexists a linear AlIn gradation between the first layer and thesubsequent layer.
 16. The transistor apparatus of claim 1, wherein theepitaxial first structure includes Al_(0.3)In_(0.7)Sb, wherein thesubsequent layer is up to a 30th layer after the first layer, whereinthe subsequent layer includes Al_(0.7)In_(0.3)Sb, and wherein thereexists an exponential AlIn gradation between the first layer and thesubsequent layer.
 17. The transistor apparatus of claim 1, wherein theepitaxial first structure includes Al_(0.3)In_(0.7)Sb, wherein thesubsequent layer is up to a 30th layer after the first layer, whereinthe subsequent layer includes Al_(0.7)In_(0.3)Sb, and wherein thereexists an exponential AlIn gradation with an inflection between thefirst layer and the subsequent layer.
 18. The transistor apparatus ofclaim 1, wherein the epitaxial first structure includesAl_(0.3)In_(0.7)Sb, wherein the subsequent layer is up to a 30th layerafter the first layer, wherein the subsequent layer includesAl_(0.7)In_(0.3)Sb, and wherein there exists an exponential AlIngradation with an inflection and an asymptote between the first layerand the subsequent layer.
 19. The transistor apparatus of claim 1,wherein the epitaxial first structure includes Al_(0.3)In_(0.7)As,wherein the subsequent layer is up to a 30th layer after the firstlayer, and wherein the subsequent layer includes Al_(0.7)Ga_(0.3)As. 20.The transistor apparatus of claim 1, wherein epitaxial first structureincludes Al_(0.3)In_(0.7)Sb, wherein the subsequent layer is up to a30th layer after the first layer, wherein the subsequent layer includesAl_(0.7)Ga_(0.3)As, and wherein there exists a linear AlGa gradationbetween the first layer and the subsequent layer.
 21. The transistorapparatus of claim 1, wherein the epitaxial first structure includesAl_(0.3)Ga_(0.7)As, wherein the subsequent layer is up to a 30th layerafter the first layer, wherein the subsequent layer includesAl_(0.7)Ga_(0.3)As, and wherein there exists an exponential AlGagradation between the first layer and the subsequent layer.
 22. Thetransistor apparatus of claim 1, wherein the epitaxial first structureincludes Al_(0.3)Ga_(0.7)As, wherein the subsequent layer is up to a30th layer after the first layer, wherein the subsequent layer includesAl_(0.7)Ga_(0.3)As, and wherein there exists an exponential AlGagradation with an inflection between the first layer and the subsequentlayer.
 23. The transistor apparatus of claim 1, wherein the epitaxialfirst structure includes Al_(0.3)Ga_(0.7)As, wherein the subsequentlayer is up to a 30th layer after the first layer, wherein thesubsequent layer includes Al_(0.7)Ga_(0.3)As, and wherein there existsan exponential AlAs gradation with an inflection and an asymptotebetween the first layer and the subsequent layer.
 24. A computing systemwith a transistor device comprising: a semiconductive die, and in thesemiconductive die: a semiconductive substrate; an epitaxial firststructure of a semiconductive first type disposed on the semiconductivesubstrate; and an epitaxial second structure disposed on the epitaxialfirst structure, wherein the epitaxial second structure is a compositeincluding a first layer of a semiconductive second type disposed aboveand on the epitaxial first structure and a subsequent layer of asemiconductor subsequent type disposed above the first layer, whereinthe first layer is chemically more similar to the epitaxial firststructure than to the subsequent layer; and external memory coupled tothe semiconductive die.
 25. The computing system of claim 24, whereinthe computing system is part of one of a cellular telephone, a pager, aportable computer, a desktop computer, and a two-way radio.
 26. Atransistor apparatus comprising: a semiconductive substrate; anepitaxial first structure of a semiconductive first type disposed on thesemiconductive substrate; and an epitaxial second structure disposed onthe epitaxial first structure, wherein the epitaxial second structure isselected from graded AlInSb starting with Al_(0.3)In_(0.7)Sb and endingwith Al_(0.7)In_(0.3)Sb, and graded AlGaAs starting withAl_(0.3)Ga_(0.7)As and ending with Al_(0.7)Ga_(0.3)As; wherein theepitaxial second structure is a composite including a first layer of asemiconductive second type disposed above and on the epitaxial firststructure and a subsequent layer of a semiconductive subsequent typedisposed above the first layer, wherein the subsequent layer is up to a30th layer after the first layer, and wherein the first layer ischemically more similar to the epitaxial first structure than to thesubsequent layer.
 27. The transistor apparatus of claim 26, wherein theepitaxial second structure includes a non-linear composition gradient.28. A transistor apparatus comprising: a semiconductive substrate; anepitaxial first structure of a semiconductive first type disposed on thesemiconductive substrate; and an epitaxial second structure disposed onthe epitaxial first structure, wherein the epitaxial second structure isselected from graded In_(x)Ga_(1-x)Sb, graded In_(x)Ga_(1-x), and gradedIn_(x)Sb_(1-x); wherein the epitaxial second structure is a compositeincluding a first layer of a semiconductive second type disposed aboveand on the epitaxial first structure and a subsequent layer of asemiconductive subsequent type disposed above the first layer, whereinthe subsequent layer is up to a 30th layer after the first layer, andwherein the first layer is chemically more similar to the epitaxialfirst structure than to the subsequent layer.
 29. The transistorapparatus of claim 28, wherein the epitaxial second structure includes anon-linear composition gradient.